Potassium 40, Rubidium 87

【Potassium 40, Rubidium 87】

　Potassium-40 (40K) and Rubidium-87 (87Rb) are discussed as natural radionuclides.

Both of these nuclides have existed since the beginning of the earth, just like uranium.

Potassium 40 (⁴⁰K)

　Potassium-40 (40K) is dissolved in seawater with stable Potassium-39 (39K).

　The proportion of potassium-40 (40K) in total potassium is 0.0117%, which is a significant amount for a radionuclide. It is always present in a constant proportion to stable potassium everywhere on earth. Therefore, we humans also contain a considerable amount of potassium 40 (⁴⁰K), and in terms of becquerels, an adult weighing 60 kg contains several thousand becquerels.

　Of course, seawater also contains a significant amount of ⁴⁰K, 12 becquerels (Bq)/liter. Since cesium 137 (¹³⁷Cs) is 0.001 to 0.002 becquerel/liter, the becquerel concentration is 1000 times higher.

　Potassium is one of the salts of seawater, so it moves in line with the salinity distribution.

Rubidium 87 (⁸⁷Rb)

　Rubidium (Rb) is one of the alkali metal elements with atomic number 37. There are two types of naturally occurring rubidium: Rubidium 85 (⁸⁵Rb), which is stable with a natural abundance of 72.2%, and Rubidium 87 (⁸⁷Rb), which is present at 27.8%. ⁸⁷Rb is a radionuclide with a terrifyingly long half-life of 49.2 billion years, but seawater has a becquerel concentration of 0.11 becquerel (Bq)/liter, similar to that of uranium-238 (²³⁸U), and potassium-40. Similarly, it moves in the same way as salt in seawater.

天然放射性核種natural radionuclides

カリウムは海水に非常に溶けやすく、主要元素のひとつ

Potassium is very soluble in seawater and is one of the main elements.

海水中では塩分と同じ分布 Same distribution as salinity in seawater

約about　半減期half-life　億年x100 million years

K同様に海水に溶けやすいEasily soluble in seawater like K

【Digression: Bismuth-209 was actually a radionuclide】

To digress a little, the ⁸⁷Rb nuclide has a very long half-life, but there are actually nuclides with even longer half-lives. It is bismuth 209 (²⁰⁹Bi). Until recently, bismuth-209 (209Bi) was considered to be one of the heaviest stable nuclides, but it was found to undergo alpha decay after a half-life of 1,900 quadrillion years. Lead 208 (208Pb) is the heaviest stable class in existence.

　Judging from this half-life, 209Bi has hardly decayed since the universe was born (about 13.8 billion years ago), but 209Bi will have halved by the time the galaxy in this universe ends.